This invention relates to the application of an aluminum-containing coating on a surface, and in particular, to the application of such a coating from an aluminum-containing slurry onto the internal surfaces of a gas turbine airfoil.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. In any event, the hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2150xc2x0 F.
Many approaches have been used to increase the operating temperature limits of the airfoil portions of turbine blades and vanes to their current levels. For example, the composition and processing of the base materials themselves have been improved, and a variety of solidification techniques have been developed to take advantage of oriented grain structures and single-crystal structures.
Physical cooling techniques may also be used. In one technique, internal cooling passages are present in the interior of the turbine airfoil. Air is forced through the cooling passages and out openings at the external surface of the airfoil, removing heat from the interior of the airfoil and, in some cases, providing a boundary layer of cooler air at the surface of the airfoil.
The surfaces of the internal cooling passages may be protected with a diffusion aluminide coating, which oxidizes to an aluminum oxide protective scale that inhibits further oxidation of the internal surfaces. A number of techniques for applying the internal diffusion aluminide coating are known, including chemical vapor deposition, vapor-phase aluminiding, and above-the-pack techniques. These approaches have the drawback that they also coat other exposed surfaces. Surfaces which are not to be coated may sometimes be protected by masking, but masking may not be practical in many circumstances.
In another technique, a slurry coating containing a source of aluminum and other components is applied to the internal surface. The slurry coating is chemically reacted to deposit aluminum on the internal surface. Slurry coating has the advantage that the spatial extent of the aluminum-containing coating may be limited to specific areas such as the internal surfaces. However, existing slurry coating techniques have the drawback they may leave undesirable contamination on the blade in the form of decomposition by-products.
There is therefore a need for an improved approach to the depositing of aluminum-containing coatings on specific areas of surfaces, particularly the internal surfaces of articles such as gas turbine airfoils. The present invention fulfills this need, and further provides related advantages.
The present invention provides a slurry-based approach for coating surfaces of articles with an aluminum-containing coating. It is particularly well suited for coating the internal surfaces of articles, such as the internal surfaces of the passages within hollow airfoils of gas turbine blades and vanes. The present approach has the advantages of conventional slurry-coating processes. Additionally, the slurry is formulated to facilitate the removal of residual, excess coating material from the surfaces following the coating procedure.
A method of coating comprises the steps of providing an article having an article surface to be coated, and providing a coating slurry comprising a mixture of a carrier component comprising water and an inorganic gel former, a source of aluminum, optionally a halide activator, and an oxide dispersant. The inorganic gel former is preferably a swelling clay such as a montmorillonite clay, and most preferably a hectorite clay or a bentonite clay. The coating slurry is applied to the article surface and thereafter dried on the article surface to remove the water therefrom. The step of drying is preferably accomplished by heating the coating slurry on the article surface to a temperature of from about 180xc2x0 F. to about 950xc2x0 F., most preferably from about 180xc2x0 F. to about 250xc2x0 F., in air, for a time of from about 2 to about 48 hours. The method further includes heating the article surface with the dried coating slurry thereon to form an aluminum coating bonded to the article surface. The heating is preferably accomplished by heating to a temperature of from about 1700xc2x0 F. to about 2100xc2x0 F. for a time of from about 1 to about 16 hours, in an inert or reducing atmosphere. Optionally but desirably, the excess coating material is thereafter removed from the article surface.
The article is preferably an airfoil of a gas turbine blade or vane. In one case, the airfoil is hollow with internal passages therethrough. The step of applying is accomplished by injecting the coating slurry into and filling the internal passage of the article.
The source of aluminum is preferably aluminum, a chromium-aluminum alloy, a cobalt-aluminum alloy, a titanium-aluminum alloy, an iron-aluminum alloy, an aluminum-vanadium alloy, an aluminum-manganese alloy, or mixtures thereof. The halide activator, when used, is preferably AlF3, NH4F, AlCl3, NH4Cl, CrCl3, CrCl2, NaCl, FeCl2, FeCl3, CrF2, CrF3, or mixtures thereof. The oxide dispersant is preferably alumina, but other oxides such as yttria, zirconia, chromia, and hafnia, and mixtures thereof, may be used.
Preferably, the source of aluminum is from about 1 to about 50 percent by weight of the total weight of the source of aluminum, the halide activator, and the oxide dispersant; the halide activator is from about 0.5 to about 10 percent by weight of the total weight of the source of aluminum, the halide activator, and the oxide dispersant; and the oxide dispersant is from about 50 to about 99 percent by weight of the total weight of the source of aluminum, the halide activator, and the oxide dispersant. Most preferably, the source of aluminum is a cobalt-aluminum alloy having about 50 percent by weight cobalt, balance aluminum, present in an amount of from about 28 to about 35 weight percent of the total weight of the source of aluminum, the halide activator, and the oxide dispersant. Most preferably, the halide activator is AlF3, present in an amount of from about 4 to about 6 percent by weight of the total weight of the source of aluminum, the halide activator, and the oxide dispersant. The inorganic gel former is from about 1 to about 6 percent by weight of the total weight of the water and the inorganic gel former. The source of aluminum, the halide activator, and the oxide dispersant together constitute from about 30 to about 70 percent by weight of the mixture of the carrier component, the source of aluminum, the halide activator, and the oxide dispersant.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.